This invention relates to die lips for use with extrusion dies of the type adapted to produce sheets of a thermoplastic polymer. It relates particularly to lips adapted for use in extrusion processes where it is important to maintain a careful control over the thickness of the extruded sheet.
The extrusion of a polymer between the lips of a sheet die gives a superficially uniform product but if the gauge is monitored, minor thickness variations are often found in the cross machine direction as well as in the direction of extrusion. These can be caused by uneven lip separation along the slit, differential temperatures within the polymer melt such that the amount of swell that occurs as the polymer exits the slit varies, or the presence of minor inhomogeneities in the melt.
It is often important to produce a polymer sheet of great gauge uniformity so that the resolution of this problem of gauge variation can be a very significant commercial goal.